Control of cyclone burner

ABSTRACT

A method of operating a combustion process in a cyclone burner, after start-up thereof, is provided. A fuel and a combustion gas is fed into a combustion chamber of the cyclone burner. The velocity of the combustion gas is kept between a lower and an upper limiting gas velocity. The stoichiometric condition (sub- or over-stoichiometric) is maintained by controlling the amount of fed oxygen to the amount of fed fuel. A shift is made to the other stoichiometric condition while preventing the combustion gas from obtaining a velocity outside the range defined by the lower and upper limiting gas velocity.

TECHNICAL FIELD OF THE INVENTION

The present invention relates to a method of operating a combustionprocess in a non-slagging cyclone burner, after start up thereof.

BACKGROUND ART

A pre-heat or furnace burner of the cyclone type can be described as an“adiabatic” circular burner having a combustion chamber into whichcombustion gas, such as air, is introduced tangentially to form aswirling flow. Fuel particles are introduced into the gas flow and bythe centrifugal force acting on them they will be transported along thechamber wall. The fuel in a cyclone burner preferably comprises groundparticles, but in comparison to a free standing solid fuel burner, thedemand for fine material is much lower.

In many applications the temperature inside the cyclone burner is sohigh that the fuel ash melts and forms a slag, which must be continuallywithdrawn from the burner. This is typically the case when it is used tofire coal. In other applications, typically wood combustion, thetemperature is controlled so that melted ash—stickiness—is avoided.

In most applications, the cyclone burner is refractory lined, preventingcorrosion and minimizing heat losses. In combination with a high thermaldensity this leads to an approximately adiabatic temperature within theburner.

In many applications it is desirable to maintain the temperature withina certain temperature range in order to obtain a satisfactory carbonburnout while avoiding the drawbacks, such as the above mentionedstickiness, at high temperatures. The highest temperature is reachedjust below stoichiometric condition, i.e. the condition when the oxygenof the combustion gas or air added equals the amount for completelycombusting the fuel. If less oxygen is added, i.e. sub-stoichiometriccondition, the temperature will be lower, and the same applies if moreoxygen is added, i.e. over-stoichiometric condition, since the excessoxygen will serve as a cooling medium. This is illustrated in appendedFIG. 1.

The turndown ratio, i.e. the maximum to minimum operable fuel load ratiofor a given cyclone burner, is limited by the demand of particlecirculation and by extensive particle carryover (shortcutting). In otherwords, the gas flow or the velocity of the gas should be above a lowerlimit in order to entrain the fuel particles whilst avoidingdisentraining them due to gravitational and frictional forces, andshould also be below an upper limit in order to avoid particles exitingfrom the combustion chamber before being fully combusted.

The slagging cyclone burner is the most common application. They areoperated in an over-stoichiometric condition, the main reason being toavoid a corrosive environment at reducing conditions when firing coals.Typically a turndown ratio of about 2:1 is possible. A slagging cycloneburner is used for complete melting of ash particles, which are mainlywithdrawn as slag. In contrast, a non-slagging cyclone burner isoperated at such conditions that severe slagging will not occur insidethe burner. The ash is thereby mainly withdrawn as solid fly ashparticles. Non-slagging cyclone burners can be operated under eithersub- or over-stoichiometric conditions, although sub-stoichiometric isthe most common. Typically, a turndown ratio of 4:1 is possible.Operation under sub-stoichiometric conditions is preferred because theburner can be built more compactly. The specific volume flow of gasesthrough the cyclone burner (m³/kg_(fuel)) can be regarded asapproximately proportional to the stoichiometric ratio and thus a higherthermal load is possible under a sub-stoichiometric condition.

The prior art provides little controllability as regards the combustionprocess of cyclone burners, and it is difficult to achieve a largerturndown ratio than 4:1 while operating in the desired temperaturerange. The main reasons for this are because the retention time of thefuel particles inside the combustion chamber is limited at high gas flowor because the circulation in the combustion chamber becomesinsufficient at low gas flow. One possible solution for obtaining alarger turndown ratio would be to provide a longer burner. However, sucha construction would be costly, bulky and demand a lot of space.Furthermore, a longer burner would provide considerable layoutdifficulty if it was to replace a conventional existing burner.

SUMMARY OF THE INVENTION

An objective of the present invention is to provide a method thatenables enhanced controllability and adjustability of a compactnon-slagging cyclone burner.

Another objective of the present invention is to provide a method thatincreases the possible turndown ratio for a given cyclone burner.

These and other objectives, which will become apparent from thefollowing description, are achieved by means of a method as defined inthe accompanied claims.

The invention is based on the insight that by shifting betweensub-stoichiometric and over-stoichiometric conditions in one and thesame zone of a combustion chamber of a non-slagging cyclone burner it ispossible to obtain increased adjustability and larger turndown ratiothan in the prior art.

Commonly, it is desirable to keep the temperature in the combustionchamber of the cyclone burner within a limited temperature range. Thelower the temperature in the combustion chamber, the slower combustionrate of char particles (remainder after pyrolysis) obtained, and therebyalso char accumulation within the burner resulting in possibly a loweroutput from the cyclone burner. Suitably, the lower limit of thetemperature range is at least 700° C., and preferably 900° C. However,under certain circumstances, such as for a specific fuel material thelimit may be even lower, such as 600° C. The upper limit of thetemperature range depends inter alia on melting and sticking of theburned fuel. Suitably, the upper limit of the temperature range is atmost 1300° C., and preferably 1100° C. However, under certaincircumstances, such as for a specific fuel material the limit may beeven higher, such as 1400° C. This means that the amount of combustiongas should be controlled in relation to the amount of fuel present inthe combustion chamber in order to keep the temperature within a desiredrange. In other words according to at least one embodiment of theinvention, one of the two stoichiometric conditions: sub-stoichiometriccondition and over-stoichiometric condition, is maintained bycontrolling the amount of fed oxygen to the amount of fed fuel.

Thus, if the load, i.e. the amount of fuel fed into the combustionchamber is decreased, then the combustion gas flow may also be decreasedin order to maintain the same stoichiometric condition. The lowestpossible gas flow or gas velocity for keeping the fuel particlescirculating, will therefore normally set the lower limit of the load. Wehave realized that if the cyclone burner is operated undersub-stoichiometric conditions, it is possible to decrease the load notonly to the load limit at which the gas flow would be on the border ofbeing insufficient for the circulating motion, but also to an even lowerload by shifting to over-stoichiometric condition at said load limit.This means that excess combustion gas is suddenly provided allowing theload to be reduced considerably. Both sub- and over-stoichiometricstoichiometric conditions may keep the temperature within the desirabletemperature range.

As mentioned previously, the operation of a cyclone burner is limited bya) a minimum or lower limiting gas velocity to ensure that the fuelparticles are circulated and b) a maximum or upper limiting gas velocityset by the limit where carryover of unburned particles becomes too high.For a given cyclone furnace and a given fuel, it is possible to chooseeither to operate in an over-stoichiometric condition with a relativelylow maximum load, or to operate in a sub-stoichiometric condition with arelatively high minimum load. By combining the operational modes theturndown ratio can be increased.

According to one aspect of the invention, a method of operating acombustion process in a cyclone burner is provided. According to themethod fuel is fed into a cylindrically shaped combustion chamber of thecyclone burner and an oxygen-containing combustion gas, such as air, isintroduced with a tangential velocity component into said combustionchamber so as to provide at least partial circulation of the fuel alongthe chamber wall, for the fuel to be gasified or combusted. A lowerlimiting gas velocity and an upper limiting gas velocity is defined forsaid combustion gas. The velocity of the combustion gas is held betweensaid limiting gas velocities. Either a sub-stoichiometric condition oran over-stoichiometric condition is maintained in the combustion chamberby controlling the amount of fed oxygen to the amount of fed fuel. Themethod further comprises shifting to the other one of said twostoichiometric conditions while preventing the combustion gas fromobtaining a velocity outside the range defined by the lower limiting gasvelocity and the upper limiting gas velocity.

This means that regardless of the shifting direction, i.e. from sub- toover-stoichiometric condition or vice versa, the velocity of thecombustion gas will be no lower than the lower limiting gas velocity andno higher than the upper limiting gas velocity. This applies to bothbefore and after the act of shifting from one stoichiometric conditionto the other, and also during the actual shifting.

For a given temperature in the combustion chamber, said temperaturedefines together with said limiting gas velocities, a possibletransition region, i.e. a range of fuel loads for which transition orshifting from one of the two stoichiometric conditions to the other oneis possible in accordance with the teachings of at least one embodimentof the present invention. The minimum fuel load and the maximum fuelload for said range is dependent on the temperature.

It has been found that by mixing recirculated flue gas with theoxygen-containing combustion gas prior to feeding the combustion gasinto the combustion chamber, the possible transition region is expanded.In other words, for each given temperature the addition of recirculatedflue gas to the oxygen-containing combustion gas will result in a lowerminimum fuel load than what would be the case without the addition ofthe recirculated flue gas.

The addition of recirculated flue gas affects both the sub- and overstoichiometric conditions. The turndown ratio under sub-stoichiometricconditions can be further extended if recirculated flue gases are mixedwith the combustion gas prior to providing the combustion gas to thecombustion chamber. The effect is twofold. Firstly, the recirculatedflue gas increases the gas flow without increasing the heat releasedfrom the fuel. The stoichiometric ratio is dependent on the amount ofoxygen containing-gas. Since some of this oxygen-containing gas may bereplaced by essentially non-oxygen-containing flue gas (or having verysmall amount of oxygen) a sub-stoichiometric condition will beobtainable for an even lower load than in the case when no flue gas isrecirculated, without compromising the circulating effect. Thus, theminimum limit of gas flow is reached at a lower load. Secondly, therecirculated flue gas serves as ballast. Additional oxygen-containinggas, such as combustion air, is thus demanded in order to release moreheat from the fuel thereby maintaining the temperature, and, in otherwords, the stoichiometric ratio is displaced somewhat closer to 1. Thismeans that the minimum limit is reached at a further lower load.

Under over-stoichiometric conditions the added flue gas will partlyreplace excess combustion air. The flue gas will work as a ballast,which means that one and the same amount of fuel will heat a largermass, thereby enabling the use of less combustion air for cooling. Inthe case that the total gas flow remains the same, the benefit is thatthe oxygen concentration will decrease. Thus, less nitrogen oxide isformed.

The main effect of using recirculated flue gas is that the load spanwithin which it is possible to operate under sub-stoichiometricconditions is increased.

As an alternative to recirculated flue gas, it would be possible toobtain a similar result, i.e. expanding the possible transition region,by mixing the combustion gas with any inert gas or a gas containing alower percentage of oxygen.

While it is possible to vary the amount of combustion gas (such as air)in order to control the temperature in the combustion chamber, analternative is to use recirculated flue gas (or inert gas or lowoxygen-containing gas) for controlling the temperature in the combustionchamber. This is advantageous when it is desirable to maintain apredefined stoichiometric ratio, wherein the temperature is controlledby varying the amount of recirculated gas added to the combustion gas.The gas velocity is kept within predefined limits.

According to at least one embodiment of the invention the stoichiometricconditions are controlled without mixing any additional inert orrecirculated flue gas with the combustion gas. In this case it ispossible to maintain an essentially constant stoichiometric ratiobetween the oxygen and the fuel non-equal to 1, i.e. at one of the twostates: sub-stoichiometric and over-stoichiometric, by controlling theamount of fed combustion gas depending on the amount of fed fuel. Oneessentially constant stoichiometric ratio is held before the act ofshifting, and another ratio is held after the act of shifting from onestoichiometric condition to the other. Thus, if a relatively low load ispresent, i.e. a low amount of fuel is fed into the combustion chamber,an essentially constant over-stoichiometric ratio may be kept until thetime of shifting to an essentially constant sub-stoichiometric ratio,said time of shifting being inter alia dependent on the size of theload. The term essentially constant stoichiometric ratio should beunderstood to allow such a variation of the stoichiometric ratio thatprovides a temperature within a certain desired temperature range. Forinstance, merely as an illustrative example, reference is made to FIG.1, wherein for a temperature range of 1200° C.-1300° C. the(sub-)stoichiometric ratio should be around 0.4-0.45 and the(over-)stoichiometric ratio should be around 1.8-2. Thus, before andafter the time of shifting but not during the time of shifting, when theload is increased or decreased, the amount of combustion gas isincreased and decreased, respectively so as to keep the essentiallyconstant stoichiometric ratio.

There are different options for controlling the amount of combustion gasfed into the combustion chamber. Limiting factors are the lower limitinggas velocity and the upper limiting gas velocity in the combustionchamber. The velocity of the combustion gas supplied from a combustiongas inlet will essentially be maintained as the gas enters and travelstangentially in the combustion chamber, i.e. the losses may be regardedas negligible. Having that in mind, a straight forward design is toprovide a combustion gas inlet having a fixed cross-sectional area. Byincreasing or decreasing the amount of combustion gas entering thecombustion chamber, the velocity of the gas is controlled.Alternatively, one may choose to supply the combustion gas so as toachieve a fixed velocity (at a level between the limiting gasvelocities) and instead vary the opening area of the inlet. A largeopening area is used when a large flow, i.e. a large amount of gas, isdesired while a small opening area is used when a low amount of gas isdesired. The desired amount of gas depends on the amount of fuel, as hasbeen previously described. A further controlling alternative is to varyboth the cross-sectional area of the inlet and the velocity of theprovided combustion gas. Thus, in all three cases the gas flow, i.e. thevolume per unit of time, is controllable.

A speedometer or a flowmeter may be provided in the gas supply pipingfor measuring and calculating the velocity of the combustion gas.Correspondingly, measuring devices, such as speedometer or flowmeter,may be provided for calculating the amount of fuel that is fed into thecombustion chamber. Such measurements and calculations suitably serve asa basis for deciding on the time of shifting from one stoichiometriccondition to the other one.

The described method of operating a combustion process in a cycloneburner is applicable for solid, liquid or gaseous fuel. It has beenfound particularly suitable for use with solid fuels. The solid fuel isaptly some kind of biofuel. The solid fuel may be in the form ofparticles, such as wood particles, preferably wood pellets, typicallycrushed wood pellets of a diameter up to 4 mm.

When using solid fuel particles, the lowest velocity for keeping atleast a majority of the fuel particles circulating in the combustionchamber is set as said lower limiting gas velocity. The lower limitinggas velocity may also be set on the basis of the largest particle sizeof the fuel or on some other basis. For instance, some type of fuelparticles that enter the combustion chamber will rapidly release theirvolatile matter, thereby decreasing the particle density. It maytherefore be suitable in such cases to base the minimum or lowertangential gas velocity on the particle density obtained afterdevolatilisation. For wood particles this density is typically in themagnitude of 250 kg/m³, about a quarter of the particle density beforeentering the combustion chamber.

For a “lying” cyclone burner, i.e. comprising a combustion chamberhaving a central axis of symmetry extending horizontally, the lowerlimiting gas velocity is suitably set so that certain criteria are metat the top of the combustion chamber.

For a cyclone burner combustion chamber having a horizontal central axisand circular cross-section in the vertical plane, the circulating gasflow within the combustion chamber can be regarded as non-expanding, andtherefore the tangential periphery velocity equal to the gas inletvelocity.

Five forces act on the fuel particles, namely: Gravity  F_(g) = −m_(p)g${{Centrifugal}{\quad\quad}F_{c}} = {m_{p}\frac{V_{p,t}^{2}}{R}}$Friction  F_(f) = −μ  m_(p)a_(N)${{Tangential}\quad{drag}\quad F_{d,t}} = {C_{d}A_{p}\rho_{g}\frac{\lbrack {V_{g,t} - V_{p,t}} \rbrack^{2}}{2}}$${{Radial}\quad{drag}\quad F_{d,r}} = {C_{d}A_{p}\rho_{g}\frac{\lbrack {V_{g,r} - V_{p,r}} \rbrack^{2}}{2}}$wherein

-   -   m_(p)=mass of a particle    -   g=gravitational constant    -   R=radius of the combustion chamber of the cyclone burner    -   V_(g,t)=tangential gas velocity    -   V_(g,r)=radial gas velocity    -   V_(p,t)=tangential particle velocity    -   V_(p,r)=radial particle velocity    -   μ=friction factor    -   α_(N)=acceleration in normal direction    -   C_(d)=drag coefficient    -   A_(p)=cross-sectional area of a fuel particle    -   ρ_(g)=density of the combustion gas        The lower limiting gas velocity is suitably set by the situation        where a particle at the highest position (at the top) is just        prevented from falling down. This is the case when the gravity        and radial drag forces balance the centrifugal force, resulting        in zero friction. The limiting tangential particle velocity        becomes: $\begin{matrix}        {V_{p,t} = \sqrt{R\lbrack {g + {C_{d}\frac{A_{p}}{m_{p}}\rho_{g}\frac{( {V_{g,r} - V_{p,r}} )^{2}}{2}}} \rbrack}} \\        {= \sqrt{R\lbrack {g + {\frac{3}{4}\frac{C_{d}}{d_{p}}\frac{\rho_{g}}{\rho_{p}}( {V_{g,r} - V_{p,r}} )^{2}}} \rbrack}}        \end{matrix}$        The radial drag can be assumed to be negligible, and the        limiting tangential particle velocity (V_(p,t)) is expressed as:        V _(p,t) ={square root}{square root over (gR)}

However, the tangential gas velocity inside the combustion chamber mustbe greater than the limiting particle velocity. The lower limiting gasvelocity can be found by solving the following differential equation,thus determining the gas velocity securing the desired particle velocityat the top of the cyclone burner.${F_{d,t} + F_{f} + F_{g}} = {{m_{p}\frac{\delta\quad V_{p,t}}{\delta\quad t}} = {m_{p}V_{p,t}\frac{\delta\quad V_{p,t}}{\delta\quad S}}}$${{Thus}:{{C_{d}A_{p}\rho_{g}\frac{\lbrack {V_{g,t} - V_{p,t}} \rbrack^{2}}{2}} - {\mu\quad{m_{p}\lbrack {{g\quad{\cos(\varphi)}} + \frac{V_{p,t}^{2}}{R}} \rbrack}} - {m_{p}g\quad{\sin(\varphi)}}}} = {m_{p}V_{p,t}\frac{\delta\quad V_{p,t}}{\delta\quad S}}$Here φ is the angle to the vertical, i.e. 180° at the top of thecombustion chamber, and S is the distance travelled by the particlealong the periphery.

Solving for the tangential gas velocity V_(g,t) giving the desiredparticle velocity at the top V_(p,t)={square root}{square root over(gR)}, one finds that it (V_(g,t)) increases as the radius of thecombustion chamber of the cyclone burner and the particle diameterincrease.

In a “standing” cyclone burner, i.e. a combustion chamber having acentral axis of symmetry extending vertically and a circularcross-section in the horizontal plane, the forces acting on the particleare similar as for the “laying” cyclone with the addition of a verticaldrag force. However, for simplicity, both the radial and vertical forcesare considered as negligible. By assuming so, the tangential lowerlimiting gas velocity V_(g,t) is calculated by solving the followingequation (which will be further discussed in connection withaccompanying FIG. 11): $\begin{matrix}{V_{g,t} = {\sqrt{{gR}\frac{{\tan(\alpha)} - \mu}{{\mu\quad{\tan(\alpha)}} + 1}} +}} \\{\sqrt{\frac{4}{3}d_{p}\frac{\rho_{p}}{\rho_{g}}{\frac{\mu}{Cd}\lbrack {{g\quad c\quad{{os}(\alpha)}} + {g\frac{{\tan(\alpha)} - \mu}{{{\mu tan}(\alpha)} + 1}{\sin(\alpha)}}} \rbrack}}}\end{matrix}$whereas

-   -   V_(g,t)=tangential gas velocity    -   g=gravitational constant    -   R=radius of the combustion chamber of the cyclone burner    -   α=the angle to the horizontal    -   μ=friction factor    -   d_(p)=diameter of a fuel particle    -   ρ_(p)=density of a fuel particle    -   ρ_(g)=density of the combustion gas    -   C_(d)=drag coefficient

Alternatively, the lower limiting gas velocity may be determinedempirically, i.e. by doing tests for a specific cyclone burner firedwith a specific fuel. The method according to the present invention isapplicable regardless of how the lower limiting gas velocity isdetermined.

The upper limiting gas velocity is suitably set at the highest velocityallowable for minimizing the amount of unburned fuel particles leavingthe combustion chamber, said velocity being 20-50 m/s, preferably 25-40m/s, such as in the order of 30 m/s. Another definition of the upperlimiting gas velocity is 3-6 times the lower limiting gas velocity,typically 4 times.

One may expect that the separation efficiency, i.e. the tendency of theparticles to travel along the wall of the combustion chamber, wouldincrease infinitely as the tangential gas velocity is increased.However, in practice, re-entrainment of particles towards the centralaxis of the combustion chamber starts to be quite noticeable at acertain velocity due to the increased turbulence and vortex break downinside the cylindrical combustion chamber of the cyclone burner. Eventhough it is not straight forward to calculate the upper limiting gasvelocity, it is understood by experience that a typical value is in theorder of 30 m/s.

Another aspect limiting the possible upper gas velocity is the volumeconcentration of unburned fuel particles within the combustion chamber.It is the burn out time of the char (the remainder afterdevolatilization of the fuel) which is limiting. For a given temperatureand stoichiometric ratio the amount of unburned char will within thecombustion chamber of the cyclone burner be proportional to the load,and thereby also the tangential gas velocity. At a certain load theconcentration of unburned fuel particles will become so high thatre-entrainment will become quite noticeable. At over-stoichiometricconditions re-entrainment due to high tangential velocity is likely tobe the limiting factor. At sub-stoichiometric operation re-entrainmentdue to choking by fuel particles is more likely.

The procedure for determining the upper limiting gas velocity may vary,e.g. by doing tests for a specific cyclone burner fired with a specificfuel. The method according to the present invention is applicableregardless of how the upper or lower limiting gas velocities aredetermined. They have the function of limiting values. For instance,according to at least one embodiment of the invention the act ofshifting from one of the two stoichiometric conditions to the other oneis performed just before the gas reaches one of said limiting gasvelocities. According to at least one other embodiment of the inventionsaid shifting to the other one of said two conditions is performed whenthe amount of fed fuel in the current stoichiometric condition would,for the other stoichiometric condition, require such an amount ofcombustion gas which corresponds to a velocity of gas flow that iswithin the interval of the limiting gas velocities.

As has been discussed above, the method according to the presentinvention provides a turndown ratio for cyclone burners, which isconsiderably greater than what has been possible to achieve in the priorart. Even though it is desirable to keep the temperature within acertain interval, both for sub- and over-stoichiometric conditions, saidinterval can actually be quite useful for further increasing theturndown ratio. Even though a temperature range between 900° C.-1100° C.may be preferred inside the cyclone burner, the range may acceptably beextended to 700° C.-1300° C. or even more. For instance, if one canallow a higher than normal temperature during sub-stoichiometricconditions, such as close to or about 1300° C., more oxygen is neededthan usual in order to raise the temperature for the same amount ofload. Since more oxygen-containing gas is allowed to be introduced tothe cyclone burner relative to the amount of load, this means that thestoichiometric ratio is closer to 1, having the consequence that a lowerminimum load is allowed, while still introducing enough gas to keep theparticles circulating. Similarly, during over-stoichiometric conditionsa relatively lower temperature may be allowable, i.e. more oxygen inrelation to the load. This will also lead to a possible lower minimumload.

Even if it is possible to make use of varying temperatures, in manycases it may be desirable to maintain as even a temperature as possible.This may particularly apply to the actual time of shifting from sub- toover-stoichiometric ratio, and vice versa. Therefore, suitably, such ashift is performed swiftly so as to maintain the temperature level aseven as possible. This may be achieved by means of a regulating system,e.g. comprising a computer, flowmeters for the fuel and the combustiongas and valves. The system may be programmed in the following manner. Atover-stoichiometric operation a condition arises that a decreased amountof input combustion gas leads to an increase in temperature. A minimumallowed stochiometric ratio, above 1.0, is also set. Atsub-stoichiometric conditions said condition is changed to where anincreased amount of input combustion gas results in an increase intemperature, and the minimum stochiomatric ratio is replaced with anmaximum, which is beneath 1.0. At the point of shifting tosub-stoichiometric operation, the regulating system is instantaneouslygiven the new conditions, which means that the shift is obtained as fastas the valve(s) can change position. The reverse change of condition andlimit value apply when going from sub-stoichiometric toover-stoichiometric operation.

From the above description it should now be clear that the methodaccording to at least one embodiment of the present invention enables achange between gasification (i.e. sub-stoichiometric condition) athigher loads and combustion at lower loads. The invention allows this tobe performed during operation of the cyclone burner, and not only duringstart-up thereof. Furthermore, as a difference to other prior artburners which may simultaneously be operated with sub-stoichiometricconditions in one zone and over-stoichiometric conditions in anotherzone, the present method makes it possible to utilize one and the samezone of a cyclone burner for shifting between the two differentstoichiometric conditions.

It should also be clear that the inventive idea enables an increasedturndown ratio (the relationship between the largest and smallestpossible load to be fired in the cyclone burner). This may be usefule.g. when it is desirable to change the output to a furnace connected tothe cyclone burner, typically in a district heating plant (up to 30-50MW) or even in a domestic boiler (a couple of 100 kW). The temperaturein the burner may be kept relatively constant during operation, however,the amount of fuel, and consequently the output, may be varied e.g.depending on day or night operation. An increased turndown ratio of acyclone burner facilitates the changing between the need for more orless output. In prior art burners it may sometimes be necessary tointerrupt the operation of the burner, because it is not possible toproduce a sufficiently low output, and therefore when larger outputagain is desired, the burner has to be re-started. The present inventiveidea, however, provides a larger possible regulating range.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram illustrating the relationship between stoichiometricratio and adiabatic temperature when wood pellets are used as fuel.

FIG. 2 is a diagram illustrating the theoretical minimum particlevelocity at the top of a combustion chamber as a function of thecombustion chamber diameter.

FIG. 3 is a diagram illustrating the calculated lower limiting gasvelocity as a function of particle diameter and combustion chamberdiameter.

FIG. 4 is another diagram illustrating the calculated lower limiting gasvelocity as a function of particle diameter and combustion chamberdiameter.

FIG. 5 is a diagram illustrating the turndown ratio depending on thestoichiometric ratio and the relative gas flow.

FIG. 6 is a another diagram illustrating the turndown ratio.

FIG. 7 is a diagram illustrating the turndown ratio in the case ofrecirculated flue gases being added to the combustion gas.

FIG. 8 is another diagram illustrating the turndown ratio in the case ofrecirculated flue gases being added to the combustion gas.

FIG. 9 is yet another diagram illustrating the turndown ratio in thecase of recirculated flue gases being added to the combustion gas.

FIG. 10 is a further diagram illustrating the turndown ratio in the caseof recirculated flue gases being added to the combustion gas.

FIG. 11 illustrates forces acting on a particle in a standing cycloneburner.

DETAILED DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram illustrating the relationship between stoichiometricratio and adiabatic temperature when wood pellets are used as fuel. Thewood pellets may have a lower heating value (or net calorific value) of18.2 MJ/kg. The diagram shows that the highest temperature is obtainedfor a stoichiometric ratio of approximately 0.95. If more oxygen isprovided in relation to what is needed for complete combustion of thefuel, i.e. an over stoichiometric condition, the temperature becomeslower. For instance, a stoichiometric ratio of 2.0 results in anadiabatic temperature of 1200° C. Similarly, if less oxygen is providedso as to achieve a more sub-stoichiometric condition, the temperaturewill also become lower. For instance a stoichiometric ratio of 0.5 wouldresult in a temperature of approximately 1400° C. As describedpreviously, in order to obtain satisfactory operability, it may bedesirable to keep the temperature within a certain range. Thus, for thisparticular fuel, if it would be desirable to operate within thetemperature range of 1100° C.-1300° C., the sub- and over-stoichiometricratios would be held at approximately 0.37-0.45 and 1.8-2.25,respectively.

FIG. 2 is a diagram illustrating the theoretical minimum particlevelocity at the top portion of the combustion chamber of a lying cycloneburner as a function of the combustion chamber diameter. As has beendescribed previously, the lower limiting gas flow is set by the case inwhich a particle at the highest position (the top) of the combustionchamber is just prevented from falling down. If the radial drag isassumed to be negligible, the tangential particle velocity (V_(p,t)) isV_(p,t)={square root}{square root over (gR)}. This is illustrated inFIG. 2. For instance, a combustion chamber having a diameter of 0.3 m,0.6 m or 1.2 m would result in a minimum particle velocity at the top of1.2 m/s, 1.7 m/s and 2.4 m/s, respectively.

FIG. 3 is a diagram illustrating the calculated lower limiting gasvelocity as a function of particle diameter and combustion chamberdiameter in a lying cyclone burner. The tangential gas velocity(V_(g,t)) must be higher than the minimum particle velocity (V_(p,t)).As has been described previously, the tangential gas velocity V_(g,t)should be so high that the particle velocity at the upper position(φ=180°) in the combustion chamber of the cyclone burner is higher thanthe calculated minimum particle velocity (V_(p,t)). Using this asboundary condition the gas velocity is solved from the followingdifferential equation${{C_{d}A_{p}\rho_{g}\frac{\lbrack {V_{g,t} - V_{p,t}} \rbrack^{2}}{2}} - {\mu\quad{m_{p}\lbrack {{g\quad{\cos(\varphi)}} + \frac{V_{p,t}^{2}}{R}} \rbrack}} - {m_{p}g\quad{\sin(\varphi)}}} = {m_{p}V_{p,t}\frac{\delta\quad V_{p,t}}{\delta\quad S}}$One finds that the lower limiting gas velocity (V_(g,t)) increases asthe radius of the combustion chamber of the cyclone burner and theparticle diameter increase. This is illustrated in FIG. 3. Thehorizontal axis in the diagram represents the particle diameter in mmand the vertical axis represents the lower limiting gas velocity in m/s.Three curves are drawn, wherein the bottom curve is for a combustionchamber diameter of 0.3 m, the middle curve is for a combustion chamberdiameter of 0.6 m and the top curve is for a combustion chamber diameterof 1.2 m. For the calculations a friction factor of 0.5, a dragcoefficient of 0.44, a gas density of 0.28 kg/m³ and a particle densityof 1000 kg/m³ have been assumed. The diagram shows that for a particlediameter of e.g. 2.0 mm (e.g. crushed wood pellet) the lower limitinggas velocity is about 11 to 13 m/s depending on the size of thecombustion chamber. For a smaller particle diameter of e.g. 0.5 mm (suchas crushed pellet) the lower limiting gas velocity is as low as 6 to 8m/s.

When fuel particles enter the combustion chamber of the cyclone burnerthey will rapidly release their volatile matter. Thus, the particledensity will also decrease. It may therefore be suitable to calculatethe lower limiting gas velocity based on the particle density afterdevolatilisation. For wood particles this density is typically in themagnitude of 250 kg/m³. This is shown in FIG. 4. Thus, all input data isthe same as for the diagram shown in FIG. 3, except for the particledensity which in FIG. 4 is 250 kg/m³ instead of 1000 kg/m³. For aparticle diameter of 0.5 mm the lower limiting gas velocity is about 3to 5 m/s, which is enough for obtaining the minimum particle velocity(1.2 m/s, 1.7 m/s and 2.4 m/s) calculated above for the differentcombustion chamber diameters. If the upper limiting gas velocity, whichhas been found empirically, is about 30 m/s, the turn down ratio for agiven combustion temperature and a particle of diameter 0.5 mm would beabout 30:5, i.e. 6:1. The turn down ratio can be further extended ifalso the combustion temperature is allowed to be varied with the load.

FIG. 5 is a diagram illustrating the turndown ratio depending on thestoichiometric ratio and the relative gas flow. In this example anadiabatic temperature of about 1300° C. is presumed in the combustionchamber of the cyclone burner. The horizontal axis represents therelative load factor of the cyclone burner. The left vertical axisrepresents the stoichiometric ratio inside the combustion chamber. Theright vertical axis represents the relative gas flow inside thecombustion chamber, i.e. the ratio between the actual gas flow and theminimum gas flow, or in most cases the ratio between the actual gasvelocity and the lower limiting gas velocity.

Looking at the left part of the diagram, when a relatively small amountof fuel, i.e. a small load, is fed into the combustion chamber, acomparatively large amount of oxygen-containing combustion gas such asair is supplied so that an over-stoichiometric condition exists in thecombustion chamber. The stoichiometric ratio is kept at about 1.8, asillustrated by the dashed line L1, in order to maintain the temperatureof about 1300° C. As the load is increased the amount of combustion gasis also increased by increasing the velocity with which it is fed intothe combustion chamber, thereby maintaining an over-stoichiometriccondition. This is shown by the inclined left portion of the curve L2.In this case the stoichiometric ratio is kept essentially constant at1.8. The amount of load to be operated at over-stoichiometric conditionis determined by the lower limiting gas velocity and the upper limitinggas velocity being typically 4 times the lower one. The limiting gasvelocities are indicated by the horizontal lines L4 (lower limit) and L5(upper limit) across the diagram. Thus, as the load is increased from arelative load factor of 1 on the horizontal scale, and consequently alsothe gas velocity, one will eventually reach the upper limiting gasvelocity. This happens at 4 on the horizontal scale. A cyclone burneroperated at over-stoichiometric condition would thus be limited to aturndown ratio of 4:1.

Having reached the upper limiting gas velocity at over-stoichiometriccondition, a shifting operation is performed so as to obtain asub-stoichiometric condition, thereby allowing further increase of theload. The act of shifting to a sub-stoichiometric condition is performedby reducing the velocity of the gas before the velocity of the gasreaches or passes above said upper limiting gas velocity, as indicatedby line L6. In this case it coincides with the lower limiting gasvelocity at a sub-stoichiometric ratio of about 0.45 (at 4 on thehorizontal scale), in order to maintain the temperature at about 1300°C. Now, instead of having excess of oxygen, there is a shortage ofoxygen. The sub-stoichiometric ratio of about 0.45 is kept essentiallyconstant, as illustrated by the dashed line L7, while the amount of fuelfed into the combustion chamber is allowed to be further increased. Theamount of fuel may be increased, and therefore also the gas flow asindicated by line L8, up to such a load where the upper limiting gasvelocity is reached. This is at 16 on the horizontal scale. This meansthat if a cyclone burner would only be operated at thissub-stoichiometric ratio, a turndown ratio of 16:4, i.e. 4:1 would beobtained. By combining the two operational modes, making use of bothstoichiometric conditions, a theoretical turndown ratio of 16:1 isobtainable.

The process is reversible. Thus, it is possible to start at the rightside of the curve in FIG. 5, i.e. at a sub-stoichiometric condition. Asthe load is reduced, and therefore also the gas velocity, the lowerlimiting gas velocity is eventually reached. At this point, shifting ismade to over-stoichiometric ratio by increasing the gas velocity.Thereafter, the load may be decreased even further, until the gasvelocity is reduced, for maintaining the essentially constantover-stoichiometric ratio, to the lower limiting gas velocity.

FIG. 6 is another diagram illustrating the turndown ratio. In this case,the same fuel is used in the same combustion chamber as in FIG. 5.However, now an adiabatic temperature of about 1100° C. is desiredinside the combustion chamber. This temperature is obtained for anover-stoichiometric ratio of about 2.2, and for a sub-stoichiometricratio of about 0.38. As can be seen from FIG. 6, indicated by an arrowpointing downwards, a shift from the over-stoichiometric condition atthe upper limiting gas velocity to sub-stoichiometric condition wouldlead to a gas velocity below the lower limiting gas velocity. Similarly,a shift from the sub-stoichiometric condition, when having the lowerlimiting gas velocity, to the over-stoichiometric condition, would asindicated by the arrow pointing upwards result in gas velocity far abovethe upper limiting gas velocity. This means that in order to keep thedesired temperature and to obtain an overlap, when shifting from onestoichiometric condition to the other, the gas velocity will go past theupper and/or lower limiting gas velocities.

The difficulty illustrated in FIG. 6 is overcome by adding re-circulatedflue gases having low or no oxygen content to the combustion gas havinghigh oxygen content, such as air.

Accordingly, FIG. 7 is a diagram illustrating the turndown ratio in thecase of recirculated flue gases being added to the combustion gas. As inFIG. 6, the desired temperature in the combustion chamber is 1100° C. Afixed amount of recirculated flue gas (15% of the minimum gas flow) ismixed into the combustion gas before feeding it to the combustionchamber. The amount of recirculated flue gas is illustrated as astraight horizontal dotted line L9 at the bottom portion of the diagram.Lines corresponding to the lines in FIG. 5 have been denoted with thesame references.

As can be seen from the diagram in FIG. 7, the minimum load undersub-stoichiometric conditions is further extended now that recirculatedflue gas is applied. The recirculated flue gas increases the total gasflow without increasing the heat released from the fuel. Thus, theminimum limit of gas flow, i.e. the lower limiting gas velocity, isreached at a lower load. Furthermore, the recirculated flue gas servesas ballast. Additional combustion gas is therefore demanded in order tomaintain the desired temperature. This further increases the total gasflow, and the minimum limit is reached at a further decreased load.According to the diagram in FIG. 7 this limit is at about 3.5 on thehorizontal scale, instead of about 6 as in FIG. 6.

Under over-stoichiometric condition the added flue gas will partlyreplace excess combustion gas. Thus, the total gas flow will remain thesame as without any flue gas recirculation, but the stoichiometric ratiowill vary between about 1.8 and 2.1 as the load changes (see the dashedline L1). The benefit is that the oxygen concentration will decrease asthe load decreases, resulting in less nitrogen oxide being formed. Thus,in the diagram in FIG. 7, and in the diagram in FIG. 6, the upper loadlimit for over-stoichiometric conditions is reached at 4 on thehorizontal scale. While there is no overlap in FIG. 6, an overlap andtherefore a possible transition region PTR is obtained in the diagram ofFIG. 7 due to the extension of the minimum load under sub-stoichiometricconditions. The possible transition region PTR is defined by the lowerlimiting velocity at sub-stoichiometric condition and the upper limitingvelocity at over-stoichiometric condition. Instead of having a “thin”line L6 as shown in FIG. 5, a broader possible transition region PTR isobtained in the case shown in FIG. 7. This means that, in the case shownin the diagram, it is not necessary to wait until a limiting gasvelocity is reached in order to make the shift to the otherstoichiometric condition. Instead the shift may be performed at anearlier point when the amount of fuel is such that it does not passoutside the limit set by the other limiting gas velocity for the otherstoichiometric condition. For example, when changing fromsub-stoichiometric to over-stoichiometric condition the shift may bedone at a load amount corresponding to 4 (upper limit,over-stoichiometric) on the horizontal scale in FIG. 7, or later as fardown as a load amount corresponding to about 3.5 (lower limit,sub-stoichiometric) on the horizontal scale. It may be noted that theturndown ratio, according to the diagram in FIG. 7, is 18:1. However,since a given cyclone burner has a maximum load capacity, i.e. anaccumulation limit due to accumulation of burning devolatilisedparticles , and since the gas velocity is proportional to the load, itis quite possible that this maximum load will be reached before the gasvelocity at sub-stoichiometric conditions has reached the upper limitinggas velocity. Thus, the maximum load capacity or the accumulation limitindirectly determines the velocity limit. However, an advantage is thatthe span (turn down ratio) within which it is possible to operate atsub-stoichiometric conditions is enlarged, this being preferred from anenvironmental point of view since less nitrogen oxide is formed. This isfurther illustrated in FIG. 8.

FIG. 8 is another diagram illustrating the turndown ratio in the case ofrecirculated flue gases being added to the combustion gas. In this casethe desired temperature is 1300° C., and the diagram is drawn for thesame type of fuel in the same cyclone burner as for FIG. 5. However,FIG. 8 illustrates a 15% recirculation of flue gas in the combustiongas. Comparing the diagrams in these two Figures, it is obvious that thepossible transition region is larger when recirculated flue gas is used,since the minimum load at sub-stoichiometric conditions is moved furtherto the left in the diagram in FIG. 8. Even though it is preferred tooperate as much as possible at over-stoichiometric conditions, the useof flue gas may negatively affect the overall turndown ratio if the fluegas recirculation is not withdrawn at a higher load. In FIG. 8, forinstance, the overall turndown ratio is about 12.5:1 instead of 16:1 asin FIG. 5.

FIGS. 9 and 10 illustrate the effect of a larger part of the introducedgas being recirculated flue gas. In these examples the recirculated fluegas is 45% of the minimum gas flow, and in FIG. 9 the desiredtemperature is 1100° C., while in FIG. 10 the desired temperature is1300° C. It may be noticed that this higher recirculation of flue gasresults in a larger possible transition region. It may also be noticed,in FIG. 10, that the operational range at sub-stoichiometric combustionis nearly extended to a relative load factor of 1.

In the following, FIG. 11 will be discussed for deriving the lowerlimiting tangential gas velocity for a “standing” cyclone burner, i.e.comprising a combustion chamber having a central axis of symmetryextending vertically and a circular cross-section in the horizontalplane. In the corresponding manner as for a lying cyclone, the limitinggas velocity is set by the particles falling down vertically.

In the following it is assumed that fuel particles are not carried outthrough the outlet of the combustion chamber. For simplifying reasonsthe gas flow is described as a horizontal rotating flow (no verticaldrag force) and the radial gas flow is considered as negligible,resulting in an equilibrium of forces acting on a fuel particle 2 asillustrated in FIG. 11. The fuel particle abuts an inner wall 4 of thecombustion chamber. In order to prevent the particle from falling down,the gravitational force F_(g) is balanced by the frictional force F_(f)and centrifugal force F_(c) in the direction of the inclined plane, saidplane being inclined with an angle α from the horizontal plane H.F _(f) +F _(c) cos(α)=F _(g) sin(α)The centrifugal force F_(c) and the gravitational force F_(g) may beexpressed as: $F_{c} = {m_{p}\frac{V_{p,t}^{2}}{R}}$F_(g)=m_(p)gwherein m_(p) is the mass of the particle, V_(p,t) is the tangentialvelocity of the particle, R is the radius of the combustion chamber ofthe cyclone burner and g is the gravitational constant. The frictionalforce F_(f) is proportional to a normal force F_(N) according to:F_(f)=μF_(N)F _(N) =F _(g) cos(α)+F _(c) sin(α)$F_{f} = {\mu\quad{m_{p}\lbrack {{g\quad{\cos(\alpha)}} + {\frac{V_{p,t}^{2}}{R}{\sin(\alpha)}}} \rbrack}}$wherein μ is the friction factor or frictional coefficient. This leadsto the following relation.F _(f) +F _(c) cos(α)=F _(g) sin(α)${{\mu\quad{m_{p}\lbrack {{g\quad{\cos(\alpha)}} + {\frac{V_{p,t}^{2}}{R}{\sin(\alpha)}}} \rbrack}} + {m_{p}\frac{V_{p,t}^{2}}{R}{\cos(\alpha)}}} = {m_{p}g\quad{\sin(\alpha)}}$${{\mu\lbrack {1 + {\frac{V_{p,t}^{2}}{gR}{\tan(\alpha)}}} \rbrack} + \frac{V_{p,t}^{2}}{gR}} = {\tan(\alpha)}$${\tan(\alpha)} = \frac{\mu + \frac{V_{p,t}^{2}}{gR}}{1 - {\mu\frac{V_{p,t}^{2}}{gR}}}$Thus, the minimum tangential particle velocity will be:$V_{p,t} = \sqrt{{gR}\frac{{\tan(\alpha)} - \mu}{{\mu\quad{\tan(\alpha)}} + 1}}$From the above it is clear that it is possible to have a steeperinclination if a) the radius R is decreased, b) the tangential particlevelocity V_(p,t) is increased, or c) the frictional coefficient μ isincreased.

In order to maintain the tangential particle velocity, the tangentialdrag force F_(d,t) has to balance the frictional force F_(f). Thefrictional force is equal in all directions.$F_{d,t} = {C_{d}A_{p}\rho_{g}\frac{\lbrack {V_{g,t} - V_{p,t}} \rbrack^{2}}{2}}$wherein C_(d) is the drag coefficient, A_(p) is the cross-sectional areaof a fuel particle, ρ_(g)=density of the combustion gas andV_(g,t)=tangential gas velocity.$F_{f} = {{\mu\quad{m_{p}\lbrack {{g\quad{\cos(\alpha)}} + {\frac{V_{p,t}^{2}}{R}{\sin(\alpha)}}} \rbrack}} = {\rho_{g}A_{p}{Cd}\frac{( {V_{g,t} - V_{p,t}} )^{2}}{2}}}$Thus, the minimum tangential gas velocity will be:$V_{g,t} = {V_{p,t} + \sqrt{\frac{2\mu\quad m_{p}}{\rho_{g}A_{p}{Cd}}\lbrack {{g\quad{\cos(\alpha)}} + {\frac{V_{p,t}^{2}}{R}{\sin(\alpha)}}} \rbrack}}$Substituting the mass m_(p) with the particle density ρ_(p) times thevolume of the particle, d_(p) being the diameter of the particle, andrewriting the cross-sectional area A_(p) of the particle$m_{p} = {\rho_{p}\frac{4}{3}{\pi( \frac{d_{p}}{2} )}^{3}}$$A_{p} = {{\pi( \frac{d_{p}}{2} )}^{2}\quad{gives}}$$V_{g,t} = {V_{p,t} + \sqrt{\frac{4}{3}d_{p}\frac{\rho_{p}}{\rho_{g}}{\frac{\mu}{Cd}\lbrack {{g\quad{\cos(\alpha)}} + {\frac{V_{p,t}^{2}}{R}{\sin(\alpha)}}} \rbrack}}}$By substituting the expression for the minimum tangential particlevelocity the following equation is obtained.$V_{g,t} = {\sqrt{{gR}\frac{\quad{{\tan(\alpha)} - \mu}}{{\mu\quad{\tan(\alpha)}} + 1}} + \sqrt{\frac{4}{3}d_{p}\frac{\rho_{p}}{\rho_{g}}{\frac{\mu}{Cd}\lbrack {{g\quad{\cos(\alpha)}} + {g\quad\frac{{\tan(\alpha)} - \mu}{{\mu\quad{\tan(\alpha)}} + 1}{\sin(\alpha)}}} \rbrack}}}$The larger or heavier the particle, the larger combustion chamber radiusand higher tangential gas velocity required. Furthermore, the lowerlimiting gas velocity is increased as the angle α is increased and thefrictional coefficient is decreased.

1. A method of operating a combustion process in a non-slagging cycloneburner, after start-up thereof, comprising: feeding a fuel into acylindrically shaped combustion chamber of the cyclone burner; feedingan oxygen-containing combustion gas with a tangential velocity into saidcombustion chamber, a lower limiting gas velocity and an upper limitinggas velocity being defined for said combustion gas; keeping the velocityof the combustion gas between said limiting gas velocities; maintainingone of two stoichiometric conditions: sub-stoichiometric condition andover-stoichiometric condition, by controlling the amount of fed oxygento the amount of fed fuel; and shifting to the other one of said twostoichiometric conditions while preventing the combustion gas fromobtaining a velocity outside the range defined by the lower limiting gasvelocity and the upper limiting gas velocity.
 2. The method as claimedin claim 1, further comprising: maintaining the temperature in thecombustion chamber in the temperature range of 700° C.-1300° C., whereineach temperature point in said temperature range defines, together withsaid limiting gas velocities, a respective minimum fuel load and arespective maximum fuel load for shifting from one of the twostoichiometric conditions to the other one.
 3. The method as claimed inclaim 2, further comprising: mixing recirculated flue gases, or otherlow oxygen-containing gas or inert gas, with the oxygen-containingcombustion gas prior to feeding the combustion gas into the combustionchamber, thereby reducing said minimum fuel load undersub-stoichiometric conditions.
 4. The method as claimed in claim 2,further comprising: mixing recirculated flue gases, or other lowoxygen-containing gas or inert gas, with the oxygen-containingcombustion gas prior to feeding the combustion gas into the combustionchamber, thereby reducing, at the same total gas flow, the oxygenconcentration and thereby the formation of nitrogen oxides underover-stoichiometric conditions.
 5. The method as claimed in claim 1,wherein the act of maintaining a stoichiometric condition compriseskeeping an essentially constant stoichiometric ratio in order to controlthe temperature.
 6. The method as claimed in claim 2, wherein thestoichiometric ratio is kept within defined limits while the temperaturein the combustion chamber is controlled by the amount of saidrecirculated flue gas, or other low oxygen-containing gas or inert gasto be mixed with the oxygen-containing combustion gas.
 7. The method asclaimed in claim 1, comprising feeding said fuel in the form of solidfuel particles.
 8. The method as claimed in claim 7, comprising:controlling, for a relatively small amount of fuel being fed into thecombustion chamber, the amount of combustion gas so that anover-stoichiometric condition prevails in the combustion chamber;increasing, when the amount of fuel is increased, the amount ofcombustion gas, by increasing the velocity with which it is fed into thecombustion chamber, thereby maintaining an over-stoichiometriccondition; shifting to a sub-stoichiometric condition by reducing therelative amount of combustion gas, by reducing the velocity of thecombustion gas, before the velocity of the gas reaches said upperlimiting gas velocity or when the amount of fuel is such that asub-stoichiometric condition is obtainable that meets the criteria ofthe temperature in the combustion chamber being 700° C.-1300° C., andthe velocity of the gas being equal to or higher than said lowerlimiting gas velocity.
 9. The method as claimed in claim 8, wherein,after shifting to a sub-stoichiometric condition, the method furthercomprising: increasing, when the amount of fuel is further increased,the amount of combustion gas by increasing the velocity with which it isfed into the combustion chamber, while maintaining a sub-stoichiometriccondition.
 10. The method as claimed in claim 7, comprising:controlling, for a relatively large amount of fuel being fed into thecombustion chamber, the amount of combustion gas so that asub-stoichiometric condition prevails in the combustion chamber;reducing, when the amount of fuel is reduced, the amount of combustiongas, by reducing the velocity with which it is fed into the combustionchamber, thereby maintaining a sub-stoichiometric condition; shifting toan over-stoichiometric condition by increasing the relative amount ofcombustion gas, by increasing the velocity of the combustion gas, beforethe velocity of the gas reaches said lower limiting gas velocity or whenthe amount of fuel is such that an over-stoichiometric condition isobtainable that meets the criteria of the temperature in the combustionchamber being 700° C.-1300° C., and the velocity of the gas being equalto or lower than said upper limiting gas velocity.
 11. The method asclaimed in claim 10, wherein, after shifting to the over-stoichiometriccondition, the method further comprising: reducing, when the amount offuel is further reduced, the amount of combustion gas by reducing thevelocity with which it is fed into the combustion chamber, whilemaintaining an over-stoichiometric condition.
 12. The method as claimedin claim 7, in which said lower limiting gas velocity is the lowestvelocity for keeping at least a majority of the fuel particlescirculating in the combustion chamber.
 13. The method as claimed inclaim 7, wherein, for a cyclone burner with a combustion chamber havinga central axis of symmetry extending horizontally, the tangential lowerlimiting gas velocity V_(g,t) at the top of the combustion chamber iscalculated by solving the following differential equation:${{C_{d}A_{p}\rho_{g\quad}\frac{\lbrack {V_{g,t} - V_{p,t}} \rbrack^{2}}{2}} - {\mu\quad{m_{p}\lbrack {{g\quad{\cos(\varphi)}} + \frac{V_{p,t}^{2}}{R}} \rbrack}} - {m_{p}g\quad{\sin(\varphi)}}} = {m_{p}V_{p,t}\frac{\delta\quad V_{p,t}}{\delta\quad S}}$fulfilling the boundary condition V_(p,t)={square root}{square root over(gR)} for φ=180°. wherein μ=friction factor C_(d)=drag coefficientA_(p)=cross-sectional area of a fuel particle ρ_(g)=density of thecombustion gas φ=the angle to the vertical, i.e. 180° at the top of thecombustion chamber V_(g,t)=tangential gas velocity V_(p,t)=tangentialparticle velocity m_(p)=mass of a particle g=gravitational constantR=radius of the combustion chamber of the cyclone burner S=the distancetraveled along the periphery by the particle
 14. The method as claimedin claim 7, wherein, for a cyclone burner with a combustion chamberhaving a central axis of symmetry extending vertically, the tangentiallower limiting gas velocity V_(g,t) is calculated by solving thefollowing equation:$V_{g,t} = {\sqrt{{gR}\frac{\quad{{\tan(\alpha)} - \mu}}{{\mu\quad{\tan(\alpha)}} + 1}} + \sqrt{\frac{4}{3}d_{p}\frac{\rho_{p}}{\rho_{g}}{\frac{\mu}{Cd}\lbrack {{g\quad{\cos(\alpha)}} + {g\quad\frac{{\tan(\alpha)} - \mu}{{\mu\quad{\tan(\alpha)}} + 1}{\sin(\alpha)}}} \rbrack}}}$wherein V_(g,t)=tangential gas velocity g=gravitational constantR=radius of the combustion chamber of the cyclone burner α=the angle tothe horizontal μ=friction factor d_(p)=diameter of a fuel particleρ_(p)=density of a fuel particle ρ_(g)=density of the combustion gasC_(d)=drag coefficient
 15. The method as claimed in claim 7, in whichsaid upper limiting gas velocity is the highest velocity allowable forpreventing a large amount of unburned fuel particles from leaving thecombustion chamber, said velocity being 20-50 m/s.
 16. The method asclaimed in claim 2, wherein the act of maintaining a stoichiometriccondition comprises keeping an essentially constant stoichiometric ratioin order to control the temperature.
 17. The method as claimed in claim3, wherein the stoichiometric ratio is kept within defined limits whilethe temperature in the combustion chamber is controlled by the amount ofsaid recirculated flue gas, or other low oxygen-containing gas or inertgas to be mixed with the oxygen-containing combustion gas.
 18. Themethod as claimed in claim 7, further comprising: maintaining thetemperature in the combustion chamber in the temperature range of 700°C.-1300° C., wherein each temperature point in said temperature rangedefines, together with said limiting gas velocities, a respectiveminimum fuel load and a respective maximum fuel load for shifting fromone of the two stoichiometric conditions to the other one.
 19. Themethod as claimed in claim 18, further comprising: mixing recirculatedflue gases, or other low oxygen-containing gas or inert gas, with theoxygen-containing combustion gas prior to feeding the combustion gasinto the combustion chamber, thereby reducing said minimum fuel loadunder sub-stoichiometric conditions.
 20. The method as claimed in claim18, further comprising: mixing recirculated flue gases, or other lowoxygen-containing gas or inert gas, with the oxygen-containingcombustion gas prior to feeding the combustion gas into the combustionchamber, thereby reducing, at the same total gas flow, the oxygenconcentration and thereby the formation of nitrogen oxides underover-stoichiometric conditions.
 21. The method as claimed in claim 7,wherein the act of maintaining a stoichiometric condition compriseskeeping an essentially constant stoichiometric ratio in order to controlthe temperature.
 22. The method as claimed in claim 18, wherein thestoichiometric ratio is kept within defined limits while the temperaturein the combustion chamber is controlled by the amount of saidrecirculated flue gas, or other low oxygen-containing gas or inert gasto be mixed with the oxygen-containing combustion gas.
 23. The method asclaimed in claim 18, wherein the act of maintaining a stoichiometriccondition comprises keeping an essentially constant stoichiometric ratioin order to control the temperature.
 24. The method as claimed in claim19, wherein the stoichiometric ratio is kept within defined limits whilethe temperature in the combustion chamber is controlled by the amount ofsaid recirculated flue gas, or other low oxygen-containing gas or inertgas to be mixed with the oxygen-containing combustion gas.
 25. Themethod according to claim 7, where said solid fuel particles are crushedwood pellets having a diameter up to 4 mm.